1. Field of the Invention
The present invention relates to recording apparatus such as electrophotographic copying apparatus, and more particularly to a recording apparatus which is improved so as to produce a clear, unblurred image and/or to fix the toner with efficient use of energy.
2. Description of the Prior Art
In electrostatic recording apparatus such as electrophotographic copiers or facsimile apparatus (hereinafter referred to as "recording apparatus"), the formation of an image to be recorded is carried out by an exposure operation. It involves light reflected off a document glass plate (copy board) carrying an original through an optical system including a lens and mirrors and onto an electrostatic image support means such as a photoreceptor known in the prior art (e.g., Se or an organic photoconductor). This results in the formation on a dielectric of an electrostatic image corresponding to the original, or an electrostatic image of an original, or of an image derived from electric information signals transmitted from the outside. The electrostatic image is developed to be a visible image (toner image); the visible image is transferred onto a copy paper supplied from a given position; and the transferred image is then fixed onto the paper to thereby form a recorded image.
For convenience's sake, the process of image formation as known in the prior art will be illustrated in detail by FIG. 1 which is a schematic drawing showing the construction of an electrophotographic copying apparatus. A document G is placed on a document glass plate 1 which is arranged to be movable in reciprocating motion in the directions of the arrows. The document is illuminated scanningly through a slit (not shown) by a stationary lamp 2, and the reflected light from document G is projected through fixed mirror 3, fixed lens L, and fixed mirror 3' upon a photoreceptor 4a that is formed on the surface of a rotary drum 4, whereby an electrostatic image corresponding to the foregoing document is formed. In the present invention, the latent image-forming system comprised of the foregoing illuminating lamp, lens and mirrors will sometimes be called the "optical system" or "optical device."
The electrostatic image is converted into a toner image by any well known developing means 6, such as a magnetic brushtype developing device, arranged in the proximity of the periphery of rotary drum 4 (hereinafter called "photoreceptor drum"). At this moment, a copy paper P that has been transported from a paper feed bin 7 by roller 8 rests with its trailing end nipped by and between a pair of register rollers 9.
After that, upon the action of the above register rollers in response to a signal from movable document glass plate 1 or from photoreceptor drum 4, the copy paper P with its one-side surface brought into contact with the toner image area on the photoreceptor drum passes above image transfer means 10. During this passage, the action of the transfer means transfers the toner image onto the copy paper. In this process, both drum and paper are in an electrostatically mutually attracting condition (particularly conspicuous where the copy paper is thin and lacking in rigidness). In practice, the separation of the paper from the photoreceptor drum is accelerated by the action of a separating means 11 that is connected to an AC power supply, or the like. The separating means is allowed to be a separating claw or a separating electrode in combination with the separating claw. The trailing end-separated paper sheet is led by the action of a transport belt 16 up to a fixing device 17. The fixing device in the example has therein a thermal source element such as a halogen lamp (not shown) and is a roller-type fixing device comprised of a heat roller 17a and pressure roller 17b that revolves pressing against the heat roller. The heat and pressure by the pair of rollers 17a and 17b fuse the toner image onto the copy paper. The fused toner image-bearing copy paper is then ejected from the apparatus by a pair of ejecting rollers 18.
The means arranged around photoreceptor drum 4 are a charge-eliminating electrode 12 for eliminating the residual charge remaining on photoreceptor drum 4, a cleaning device 13 for cleaning up the residual toner on photoreceptor drum 4, an exposure-prior-to-charging lamp 14 for neutralizing the needless charge still remaining on the photoreceptor drum after the charge elimination by charge-eliminating electrode 12, and a charging electrode 5 located adjacent to lamp 14. Further, an exposure-prior-to-transfer lamp 15 is provided for facilitating the transfer onto copy paper P of the toner image formed by developing means 6 on the photoreceptor drum 4. Lamp 15 is positioned adjacent to with toner image-transfer electrode 10. In addition, photoreceptor drum 4, heat roller 17a of fixing device 17, and document glass plate 1 are adapted to be driven through means, such as a clutch (not shown), by a single motor M or a plurality of motors.
Incidentally, in conventional-type apparatus, circumferential speed V.sub.2 of heat roller 17a of fixing device 17 is usually set to be the same as circumferential speed V.sub.1 of photoreceptor drum 4. With this arrangement, however, if the apparatus is a high-speed recording machine, the surface temperature at the contact point of heat roller 17a with pressure roller 17b does not rise high enough to fix the transferred image because of the nearly continuous absorption of the heat by the copy paper. Consequently, a poor, under-fixed image is the result.
A solution to the above problem without increasing power consumption may be accomplished by setting circumferential speed V.sub.2 of heat roller 17a to be lower than circumferential speed V.sub.1 of photoreceptor drum 4. This, however, produces another problem in that the distance (hereinafter regarded as A) between the point on photoreceptor drum 4 opposite to separation electrode 11 (separating position) and the contact point of heat roller 17a with pressure roller 17b (fixing position) must be longer than the length in the transport direction of copy paper P. If it is not, that is if length B of copy paper P is longer than distance A, then even if the leading end of copy paper P is nipped by and between heat roller 17a and pressure roller 17b, the trailing end of paper P still remains attracted electrostatically and attached to photoreceptor drum 4 (still not separated). In other words, because V.sub.1 &gt;V.sub.2, the feed speed of copy paper P corresponding to V.sub.1 of the photoreceptor drum is forcibly restrained by the heat and pressure rollers rotating at only V.sub.2. As a result, the trailing end of paper P slips out of the toner image on photoreceptor drum 4, thus producing a "blurred image".
Of course, no "blurred image" problem occurs if distance A is longer than length B in the transport direction of copy paper P because the feed speed of copy paper P in the image transfer position is governed solely by circumferential speed V.sub.1 of photoreceptor drum 4.
The above blurred image problem may be solved be designing either to make distance A from separation electrode 11 to fixing device 17 (up to the paper-nipping point between the pair of rollers 17a and 17b in the exemplified drawing) longer than length B in the transport direction of a copy sheet of the largest size (e.g., A3) among those used or to make circumferential speed V.sub.2 of the pair of rollers 17a and 17b faster to be equal to circumferential speed V.sub.1. The former method, however, requires the recording apparatus to be oversize contrary to the tendency toward reducing in size the recording apparatus. The latter method requires an increase the supply of power to the thermal source element (such as a halogen lamp, not shown) for fixation in order to prevent the occurrence of possible under-fixing problems even when copy paper sheets of a large size pass in succession. Consequently this method is disadvantageous in that it results in consumption of more electricity.